1. Field of the Invention
The present invention relates to a method and an apparatus for preventing the oxidation of a lead frame in a bonding system. More particularly, the present invention concerns a method and an apparatus for preventing the oxidation of a lead frame in a bonding system such as the wire bonding of the electrodes of a semiconductor chip to a lead frame and the die bonding of a semiconductor chip to a lead frame.
2. Description of the Related Art
Generally, lead frames formed of an FeNi alloy or the like are susceptible to oxidation if they are brought into contact with the atmosphere. Therefore, when wire bonding or die bonding is performed, it is necessary to take measures to prevent the oxidation of the lead frames. In one method of preventing oxidation, a reducing gas or an inert gas is introduced into the vicinity of a lead frame being bonded.
FIG. 1 shows a partially cross-sectional view of a conventional wire bonding system which prevents oxidation of a lead frame by using such a method. A lead frame 1 with a semiconductor chip 2 mounted thereon is placed on a guide rail (not shown) and is held down by a frame holder body 6. At this stage of the process, the lead frame 1 is positioned with the semiconductor chip 2 located inside an opening 6a provided in a recess 6c of the frame holder body 6, and the lead frame 1 is held by a leg 6b of the frame holder body 6. A cover member 7 having an opening 7a corresponding to the opening 6a of the frame holder body 6 is disposed above the recess 6c of the frame holder body 6. A discharge port 8 for discharging an atmospheric gas such as a reducing gas or an inert gas is formed between the underside of this cover member 7 and the innermost surface of the recess 6c of the frame holder body 6. In addition, one end of an atmospheric gas-supplying pipe 10 is connected to the discharge port 8 via a joint 9, while the other end thereof is connected to a gas-supplying device (not shown).
During operation, the atmospheric gas is introduced from the unillustrated gas-supplying device to the discharge port 8 via the pipe 10. As shown in FIG. 2, this atmospheric gas produces a flow B which passes through the vicinity of an edge portion 6d provided around the opening 6a of the frame holder body 6 and is introduced to the lead frame 1 and the semiconductor chip 2 from above. This atmospheric gas serves to prevent the air from contacting the lead frame 1.
However, since the atmospheric gas is discharged downward from the discharge port 8 as the flow B, as shown in FIG. 2, a vacuum is created in the vicinity of the flow B of the gas. In consequence, there has been a problem in that the air flowing in through the opening 7a of the cover member 7 due to the vacuum can be caught in this flow B of the gas, as shown by the broken line C, and is brought into contact with the lead frame 1, thereby oxidizing the same.